Polymers from renewable resources have attracted an increasing amount of attention over the last two decades mainly due to two reasons: environmental concerns and the limitations of the petroleum resources. Like most polymers from petroleum feed stock, polymers from renewable resources are rarely used as functional materials in a pure state. Instead, composites comprising the polymers from renewable resources are often used to improve specific properties.
Cellulose fibers are widely used in polymeric materials to improve the mechanical properties of composite materials. Cellulose is the major substance obtained from vegetable fibers, and applications for cellulose fiber-reinforced polymers are at the forefront with a focus on renewable raw materials.
The development of synthetic polymers using monomers from natural resources provides a new direction to develop environmentally friendly biodegradable polymers from renewable sources. One of the most promising polymers in this regard is polylactic acid (PLA), because it can be made from agricultural products and is readily biodegradable.
The usual objective for preparing novel blends of two or more polymers is not to change the properties of the polymers drastically, but to capitalize on the maximum possible performance of the blend.
The use of PLA matrix reinforced with various cellulose fibers has been widely reported. The cellulose fibers have successfully improved modulus and tensile strength of the PLA matrix. However, the poor interface between hydrophobic PLA and hydrophilic cellulose fibers results in poor mechanical properties. In order to improve the interface between the PLA fibers and the cellulose-based fibers, various surface treatments have been developed, such as esterification, alkali treatment, and cyanoethylation. However, it is still a problem to produce a homogenous mixture of cellulose fibers and PLA fibers. This is especially true when manufacturing paper with high amount of bio-fiber as long bio-fibers tend to create bundles and fiber flocks. Furthermore, the strength of the cellulose will usually be reduced when mixing a biopolymer, such as PLA, with cellulose fibers. The lack of homogeneity deteriorates the structure of the obtained product and produces products with uneven properties. Therefore, it is highly important to obtain a mixture of cellulose and bio-fibers that is as homogeneous as possible. Homogeneity may be improved by using a suspension of short cellulose fibers. However, the drawback of using a suspension of short cellulose fibers is that the mechanical properties such as tear strength of the resulting material are deteriorated.
One way to increase the homogeneity of a composition as disclosed above is to add e.g. lubricants and/or surfactants to the fiber suspension. The lubricant and/or surfactants will reduce the friction between fibers and thereby reduce formation of large fiber flocks. However, the addition of lubricants and/or surfactants tend to create problems with foaming. Further methods relate to changing the surface of the cellulose fibers. However, these solutions do not fully solve the problem of providing suitable composite articles such as paper sheets comprising thermoplastic fibers such as PLA fibers and cellulose fibers having good formation, controlled air permeability, good fold and tear index when converted into different products.
WO 2009008822 discloses processes for manufacturing a composite material having reduced mechanosorptive creep. However, the process disclosed does not overcome the problem with providing homogeneity.
It is an object of the present invention to provide a process for manufacturing a composite article such as a paper sheet comprising cellulose pulp fibers and thermoplastic fibers in which the above-mentioned problems are at least partly overcome or mitigated. Further, it is an object of the invention to provide composite articles and products comprising such composite articles that overcome or mitigate the above-mentioned problems. The thermoplastic fibers may be PLA fibers.